Various types of polyethylenes are known in the art and low density polyethylene is one of the most useful. Low density polyethylene is generally prepared at high pressure using free radical initiators, or in gas phase processes using Ziegler-Natta or vanadium catalysts, and typically has a density in the range of 0.915 to 0.950 g/cm3. Typical low density polyethylene produced using free radical initiators is known in the industry as “LDPE”. LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene in the same density range, i.e., 0.915 to 0.940 g/cm3, which is linear and does not contain long chain branching is known as “linear low density polyethylene” (“LLDPE”) and is typically produced with conventional Ziegler-Natta catalysts or with metallocene catalysts. Polyethylenes having still greater density are the high density polyethylenes (“HDPEs”), i.e., polyethylenes having densities greater than 0.940 g/cm3, and are generally prepared with Ziegler-Natta catalysts. Very low density polyethylenes (“VLDPEs”) are also known. VLDPEs can be produced by a number of different processes yielding polyethylenes having a density less than 0.915 g/cm3, typically 0.890 to 0.914 g/cm3 or 0.900 to 0.914 g/cm3.
A majority of global LDPE and LLDPE demand includes film, carrying bag, and sack applications. Some examples of these applications include agricultural, multi-layer, and shrink films. LDPE, which is soft, ductile, and flexible, is additionally utilized for strong, elastic goods, such as screw caps, lids, and coatings. There remains a demand for LDPE and LLDPE in the global marketplace, and consequently there is a continued need for improvements that provide cost savings.
Some improvements include using a different catalyst system. For example, some work has been done to provide branched polymers having a density of 0.940 g/cm3 or less using metallocene compounds. JP2011089019A discloses a bridged metallocene in combination with a cocatalyst (an amine modified clay mineral, an alkyl alumoxane, or an ionized ionic compound) and an organoaluminum compound for olefin polymerization which can produce a polyolefin which possesses long chain branching, with high activity.
Other improvements have focused on the support technology. Alternative supports for metallocene and single-site catalysts have been the subject of numerous ongoing research projects. In particular, metallocenes supported on clay or ion-exchanged layered compounds have generated a great deal of interest. Olefin polymerization catalysts using clay, clay mineral, or acid/salt-treated (or a combination of both) ion-exchange layered compounds, an organoaluminum compound and a metallocene as components have been reported (see EP 0 511 665; EP 0 511 665; and U.S. Pat. No. 5,308,811). Likewise, U.S. Pat. Nos. 5,928,982 and 5,973,084 report olefin polymerization catalysts containing an acid or salt-treated (or a combination of both) ion exchange layered silicate, containing less than 1% by weight water, an organoaluminum compound, and a metallocene. Furthermore, WO 01/42320 discloses combinations of clay or clay derivatives as a catalyst support, an activator comprising any Group 1-12 metal or Group 13 metalloid, other than organoaluminum compound, and a Group 3-13 metal complex. Also, U.S. Pat. No. 6,531,552 and EP 1 160 261 report an olefin polymerization catalyst of an ion-exchange layered compound having particular acid strength and acid site densities. US 2003/0027950 reports an olefin polymerization catalyst utilizing ion-exchange layered silicates with a specific pore size distribution and having a carrier strength within a specific range.
U.S. Pat. No. 7,220,695 discloses catalyst systems comprising, inter alia, metallocene catalysts and supported activator systems comprising an ion-exchange layered silicate, an organoaluminum compound, and a heterocyclic organic compound, see Example 7 et seq.
Other background references include JP 2011-137146; US 2014-0179872; WO 2014/099307; WO 2014/099303; US 2009/0137755; US 2004/162403; US 2012/088890; US 2014/0179872; and US 2014/017988.
Accordingly, there is a need for new processes to produce polyethylene from supported high activity catalyst that have low cost methods of activation. More specifically, there is a need for new supported catalyst systems, particularly supported metallocene catalyst systems to produce polyethylene resin with low or negligible levels of long chain branching as it is known that high levels of long chain branching can be detrimental to the mechanical properties of PE film. It is further desirable that these new metallocene catalyst systems are robust and have high productivity, particularly in gas phase polymerization processes.